Along with the lighter weight and higher performance of automobiles, the load on the valve springs of the automobile engines, suspension springs of the suspension systems, clutch springs, brake springs, and other springs has increased. In recent years, high strength steel wire for spring with a tensile strength of over 2000 MPa has therefore been sought.
When producing high strength spring, the material, that is, high strength steel wire for spring, is coiled cold (cold coiling) and, furthermore, is treated by stress-relief annealing or other heat treatment and nitridation. For this reason, high strength steel wire for spring is required to be kept down in softening due to heating, that is, is required to have temper softening resistance.
Further, a spring is required to have fatigue properties, so high strength steel wire for spring is used as a material and, furthermore, nitridation and shot peening are performed to raise the hardness of the surface layer of the spring.
However, among the aspects of durability of springs, the settling properties are not determined by the hardness of the surface layer. The hardness of the base material of the spring also has a large effect. For this reason, for improving the settling properties as well, the temper softening resistance of the high strength steel wire for spring is important.
Furthermore, in the case of cold coiling, when producing the material of high strength steel wire for spring, it is possible to use oil temper treatment, induction heating treatment, etc. which enable rapid heating and rapid cooling. For this reason, it is possible to reduce the size of prior-austenite grains of the spring steel wire and obtain a spring excellent in breakage properties.
However, if steel wire for spring becomes higher in strength, in cold coiling, breakage will occur and the wire will not be able to be shaped into a spring.
To deal with this problem, some of the present inventors proposed high strength steel wire for spring controlled in retained austenite, nonmetallic inclusions, carbides, etc. (for example, see PLTs 1 to 6).
The high strength steels for spring proposed in PLTs 1 and 2 suppress the formation of retained austenite which transforms to strain induced martensite due to cold coiling and causes the workability to drop and suppress nonmetallic inclusions which become starting points of fracture.
Further, the high strength steel for spring proposed in PLT 3 controls the carbides and makes the prior-austenite finer in an attempt to achieve both strength and cold coilability.
Furthermore, the high strength steels for spring proposed in PLTs 4 to 7 control the retained austenite and carbides and make the prior-austenite finer in an attempt to achieve both strength and cold coilability. In particular, they suppress the formation of coarse oxides and carbides which form starting points of fracture and control the retained austenite in addition to the state of precipitation of carbides so as to suppress the deterioration of the fatigue properties and workability of high strength steel wire for spring.